1. Field of the Invention
The present invention relates to an optical amplifier, an optical amplifying system, a wavelength converter, and an optical communication system.
2. Description of the Related Art
Optical amplifiers have become indispensable for optical communications. In today's optical communication systems, Erbium-Doped Fiber Amplifiers (EDFAs), Raman amplifiers, and Raman amplifying systems are in practical use as optical amplifiers and optical amplifying systems for optical communication bands.
When an optical communication system is constructed by using an optical amplifier, it is desirable if the optical amplifier has excellent noise characteristics (less noise), because the less noise there is, the longer the transmission distance becomes. However, as for EDFAs, because the technology for reducing noise has matured, it is difficult to further reduce noise from the current level. In contrast, using Raman amplifiers and Raman amplifying systems can contribute to reducing noise in optical communication systems. However, it would be difficult to introduce either a Raman amplifier or a Raman amplifying system to each of all the optical fiber transmission paths that are installed.
Further, an Optical Parametric Amplifier (OPA) such as the one disclosed in International Publication No. WO 2012/121223 that utilizes a non-linear effect occurring inside an optical fiber for realizing an optical amplification is known to be able to reduce the noise than EDFAs. Further, Phase Sensitive Amplifiers (PSAs) are known as optical amplifiers that utilize a non-linear effect occurring inside an optical fiber. However, for the reasons that OPAs have a small amplification bandwidth and that the gain spectrum thereof is not flat, OPAs have not been put into practical use.
To cope with these situations, the inventors of the present invention disclosed an invention of an OPA realizing a gain spectrum that is flat and has a broadband characteristic by performing a quasi-phase-matching process (see International Publication No. WO 2012/121223, International Application No. PCT/JP2012/072255, S. Takasaka et al., “Flat and Broad Amplification by Quasi-Phase-Matched Fiber Optical Parametric Amplifier”, OFC/NFOEC 2012, OTh1C.4 (2012), and S. Takasaka et al., “FOPA with Flat 21-dB Gain and NF less than 4-dB using Alternately Concatenated Pump-Phase Shifters and HNLFs”, OFC/NFOEC 2013, JTh2A.13 (2013)).